Inspection device for masks for semiconductor lithography and method

ABSTRACT

The invention relates to an inspection device for masks for semiconductor lithography, comprising an imaging device for imaging a mask, and an image recording device, wherein one or more correction bodies which exhibit a dispersive behavior for at least one subrange of the illumination radiation used for the imaging are arranged in the light path between the mask and the image recording device. The invention furthermore relates to a method for taking account of longitudinal chromatic aberrations in inspection devices for masks, comprising the following steps: recording a specific number of images having differently defocused positions, and selecting a subset of the images and simulating a longitudinal chromatic aberration of a projection exposure apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims priority under 35 U.S.C. §120 from U.S. application Ser. No. 16/026,197, filed on Jul. 3, 2018,which is a continuation of and claims priority from German ApplicationNo. DE 10 2017 115 365.9, filed on Jul. 10, 2017. The contents of eachof these priority applications are incorporated herein by reference intheir entirety.

TECHNICAL FIELD

The invention relates to an inspection device for masks forsemiconductor lithography and to a corresponding method. Masks of thistype are generally used to image structures situated on them generallyin a manner reduced by a scale of 1:4 onto light-sensitive layers on asemiconductor substrate, a so-called wafer, by use of a projection lensof a projection exposure apparatus, a so-called scanner. This technologymakes it possible to create extremely small structures on thesemiconductor substrate and in this way to realize large scaleintegrated electronic circuits.

BACKGROUND

In order to increase the performance of the fabrication process, it isadvantageous for the masks to be subjected to an examination orinspection process as early as before their use in a scanner or atregular intervals including after removal for maintenance. For thispurpose, use is usually made of inspection devices for masks which, byuse of a microscope-like arrangement, image the masks in a magnifyingmanner and make defects identifiable in this way. Measuring systems ofthis type are usually also referred to as “aerial image measuringsystems” and are known commercially by the designations AIMS or WLCD(wafer-level critical dimension). They are usually able to simulate to acertain degree the conditions during real use of the mask in a scanner,such as, in particular, the illumination setting and the imagingsetting; for example sigma shape, numerical aperture and polarization.However, depending on the respective manufacturer or the year ofconstruction, different scanners have different so-called longitudinalchromatic aberrations that influence the behavior of the overall system.A longitudinal chromatic aberration is understood to mean the phenomenonthat, on account of the dispersive behavior of the optical materialsused, the imaging of a wafer by the projection lens lies in planes thatdeviate slightly from one another for deviating wavelengths; in otherwords: the position of the best focus of the imaging can vary with thewavelength. On account of the finite spectral width of theelectromagnetic radiation used (generally having a wavelength ofapproximately 193 nm, but radiation having a wavelength deviatingtherefrom, for example EUV radiation having a wavelength in the range of10 nm-121 nm, in particular in the region of 13.5 nm, is also used), theeffect mentioned leads to a non-negligible image unsharpness. The priorart hitherto has not yet disclosed any methods which can be used tosimulate the longitudinal chromatic aberration in an inspection devicefor masks.

SUMMARY

In a general aspect, the present invention specifies a method and adevice by use of which the longitudinal chromatic aberration of ascanner for the measurement of a mask in an inspection device for maskscan be taken into account in an improved way by comparison with thesolutions known from the prior art.

In another general aspect, an inspection device for masks forsemiconductor lithography is provided. The inspection device comprisesan imaging device for imaging a mask, and an image recording device. Oneor more correction bodies that exhibit a dispersive behavior for atleast one subrange of the illumination radiation used for the imagingare arranged in the light path between the mask and the image recordingdevice.

In another general aspect, a method for taking account of longitudinalchromatic aberrations in inspection devices for masks is provided. Themethod includes the following steps: recording a specific number ofimages having differently defocused positions, and selecting a subset ofthe images and simulating a longitudinal chromatic aberration of aprojection exposure apparatus.

The inspection device according to the invention for masks forsemiconductor lithography comprises an imaging device for imaging amask, and an image recording device. The imaging device is, e.g., amicroscope objective. The image recording device can be for example aCCD camera, CMOS-camera or linear-array camera with associated optics.According to the invention, one or more correction bodies which exhibita dispersive behavior for at least one subrange of the illuminationradiation used for the imaging are arranged in the light path betweenthe mask and the image recording device. The correction bodyadvantageously contains a medium which is dispersive in the wavelengthrange of interest, such as, for example, calcium fluoride or quartzglass.

Since the imaging device is typically configured as a transmitted-lightmicroscope, said correction body is usually situated on a side of a maskfacing away from the radiation source in the light path between the maskand the image recording device. In this case, the correction body can bearranged between the mask and the imaging device or between the imagingdevice and the image recording device. It is also possible for thecorrection body to be arranged between a radiation source and anillumination unit for the mask. Furthermore, the correction body can bearranged between an illumination unit for the mask and the mask itself.In a further variant, the correction body can be arranged within theillumination unit for the mask or within the imaging device. It goeswithout saying that it is possible that, in the case where a pluralityof correction bodies are used, the correction bodies can be arranged ata plurality of the positions mentioned above.

In this case, the correction body has the effect of simulating, onaccount of its dispersive properties, the behavior of the likewisedispersive media in projection lenses of scanners. On account of thecomparatively few, thin optical elements in the microscope objective ofthe inspection device, the longitudinal chromatic aberration, as alreadymentioned, is not adequately simulated without additional measures, andso the correction body mentioned can provide a remedy here. This is thecase in particular also because the used radiation that is used in theprojection lenses mentioned covers considerably longer optical paths inthe dispersive materials of the optical elements of the projection lensthan in the inspection device.

One advantage of the present invention here is that various longitudinalchromatic aberrations can be set, such that the different longitudinalchromatic aberrations of diverse projection systems can be taken intoaccount.

In this case, the dispersive behavior of the correction body need notnecessarily be static, but rather can be configured as dynamicallyselectable by external influencing. In this regard, firstly, in a mannerknown per se, the dispersion behavior of a correction body can beinfluenced for example by the action of a mechanical stress. Likewise,the presence of an electric, magnetic or electromagnetic field in theregion of the correction body can also influence the dispersiveproperties thereof in the desired way. Furthermore, it is possible toinfluence the dispersive behavior of the correction body by use ofthermal stresses, e.g. by use of a heating element or a Peltier elementor by use of the pressure or the gas composition of the environment inwhich the correction body is arranged. It goes without saying that it isconceivable to utilize all the effects mentioned in parallel or elsesimultaneously for influencing the dispersive behavior of the correctionbody by use of corresponding configuration of the device according tothe invention.

Furthermore, there is the possibility of varying the influence of thedispersive material of the correction body by altering the spatialorientation, position or shape of the correction element. In thisregard, by way of example, a correction body of variable thickness canbe used. This can be achieved, for example by use of a wedge-shapedcorrection body that can be displaced transversely with respect to thelight path. Furthermore, two wedges sliding on their respective wedgesurfaces can also be used.

All the measures mentioned above have in common the advantage that, byuse of a suitable choice of the constitution of the material of thecorrection body and/or by use of the suitable thermal, mechanical orelse electrical driving thereof, a simulation of the conditions in awide variety of scanners can be achieved and the behavior of the masksto be inspected in the respective target systems can thus be bettercalculated in advance.

A method according to the invention that is alternatively oradditionally usable for taking account of longitudinal chromaticaberrations in inspection devices for masks is described below.

One possible advantage of this method is that the effect of thelongitudinal chromatic aberration can be explicitly measured. This canbe used to perform further adaptations and/or optimizations. This canconcern both lithography parameters such as exposure settings or etchingprocesses and design stipulations for the mask.

In a first step, firstly a focus stack is measured; that is to say thata specific number of images having a differently defocused position arerecorded. These images can then be used to simulate a longitudinalchromatic aberration, which in its nature likewise acts like a focusaberration. In this regard, by way of example, it is possible to measure5 planes at a distance of 100 nm with respect to the mask. Withknowledge of the line width of the used radiation and the longitudinalchromatic aberration of the projection exposure apparatus, from theindividual images of the focus stack it is then possible to select thoseimages which are closest to that image which would correspond to theimage in the scanner. The longitudinal chromatic aberration of thescanner can then be simulated by suitable interpolation and, ifappropriate, weighting of the images.

The step size of the defocusing can also be configured fixedly, e.g. bedivided into a main step size and a secondary step size. In this regard,by way of example, it is possible to choose a main step size of 0.8-1.2,in particular of 1, μm with respect to the mask and a secondary stepsize of 80-120, in particular of 100, nm with respect to the mask.

In other words, the main step size addresses focus aberrations in thescanner which are process-specific, that is to say stem for example froma wafer flexure or the situation where the mask is not situated at thebest focus. These—usually situation-dependent—focus aberrations add upto form the known longitudinal chromatic aberration of the scannerrespectively considered.

The secondary step size makes it possible to further improve theaccuracy of the assignment of a defocusing of the measuring microscopeto defocusings of the mask or of the wafer (as a sum of scanner-specificlongitudinal chromatic aberration and situation-dictated deviation fromthe best-focus position in the scanner) for every assumed situation inthe scanner in particular by use of interpolation.

With the aid of the main step size, it is then possible to assess theprocess window (structure width versus defocus depending on the exposuredose) of structures, and/or the imaging behavior of defects.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments and variants of the invention are explained ingreater detail below with reference to the drawing. In the figures:

FIG. 1 shows a schematic illustration of the longitudinal chromaticaberration of an optical lens element.

FIG. 2 shows a schematic illustration of the construction of acorrection body for the targeted influencing of the dispersive behaviorof an inspection device for masks.

FIG. 3 shows a schematic illustration of the inspection device accordingto the invention for masks with possible arrangements of the position ofa correction body for the targeted setting of a specific longitudinalchromatic aberration.

FIG. 4 shows one variant for mechanically influencing a correction body.

FIGS. 5A and 5B show two variants for influencing a correction body byuse of electromagnetic fields and waves.

FIGS. 6A and 6B show two possible embodiments for setting the length ofthe path of the optical radiation within a correction body.

FIG. 7 shows one embodiment for changing the spatial orientation andposition of a correction body.

FIG. 8 shows one variant for influencing the correction body by way ofthe gas pressure and the gas composition of the environment.

FIGS. 9E, 9F, and 9G show three variants for influencing a correctionbody by use of magnetic fields.

FIG. 10 shows one embodiment for thermally influencing a correctionbody.

FIG. 11 shows one variant for changing the spectral properties of theradiation source of the inspection device for masks for the targetedsetting of a longitudinal chromatic aberration.

FIG. 12 shows a schematic illustration of a method for simulating alongitudinal chromatic aberration by use of recording a focus stack.

DETAILED DESCRIPTION

FIG. 1 illustrates a schematic illustration of the longitudinalchromatic aberration of an optical lens element 1 on account of thechromatic aberration. The electromagnetic radiation 2 having a bandwidththat is finite, but by its nature different than zero, is refracted tovarying extents within the optical lens element 1 on account of thewavelength-dependent refractive index n of the optical lens element 1,thus resulting in different focal planes F1, F2, F3. By way of example,the illustration shows three wavelengths L1, L2, L3 of theelectromagnetic radiation 2 with the corresponding focal planes F1, F2,F3, wherein L1<L2<L3 holds true. On account of the wavelength dependenceof the refractive index n and thus the focal planes F1, F2, F3, this isreferred to as a longitudinal chromatic aberration. A feature of theinvention to integrate this actually undesired optical aberration in atargeted manner within an inspection device for masks in order to beable to simulate various projection exposure apparatuses withcharacteristic longitudinal chromatic aberrations. In this case,according to the invention, one or more correction bodies are used forinfluencing the dispersive behavior.

FIG. 2 illustrates, in a schematic illustration, the construction of oneexemplary correction body 21 for the targeted influencing of thechromatic aberration. In this case, the correction body 21 comprises anoptical dispersion medium 3, for example calcium fluoride or quartzglass, wherein the optical axis OA of an inspection device for masks 4extends through the dispersion medium 3. Via a dispersion controlmechanism 22, the dispersive property of the dispersion medium 3 of thecorrection body 21 is influenced in a targeted manner in order togenerate a specific longitudinal chromatic aberration. In this case, theoptical medium 3 can be influenced statically and also dynamically. Forthe targeted control and regulation of the longitudinal chromaticaberration, the dispersion control mechanism 22 is controlled via anelectronic control unit 23.

FIG. 3 shows a schematic illustration of the device according to theinvention of an inspection device for masks 4 with possible positions(P1, P2, P3, P4, P5, P6) of one or more correction bodies 21 for thetargeted setting of a specific longitudinal chromatic aberration. Theinspection device for masks 4 comprises a radiation source 5 forgenerating electromagnetic illumination radiation 6 along the opticalaxis OA of the inspection device for masks 4. In this case, by way ofexample, an electromagnetic illumination radiation 6 having a wavelengthof 193 nm is conceivable. The electromagnetic illumination radiation 6is guided via an illumination unit 7 onto the mask 8 to be inspected. Byuse of the imaging device 9, the electromagnetic illumination radiation6 is imaged onto a camera 10 and detected there, wherein the camera 10can be configured for example as a CCD, CMOS or linear-array camera. Therecorded signal is subsequently processed electronically in a dataprocessing unit (not illustrated in the figure).

The possible positions P1, P2, P3, P4, P5, P6 of one or more correctionbodies 21 are in particular: position P1 between radiation source 5 andillumination unit 7, position P2 within the illumination unit 7,position P3 between illumination unit 7 and mask 8, position P4 betweenmask 8 and imaging device 9, position P5 within the imaging device 9,and position P6 between imaging device 9 and camera 10.

A plurality of correction bodies 21 can also be arranged for eachposition. It is also conceivable for a system comprising a plurality ofcorrection bodies 21 to be integrated at different positions within theinspection device for masks 4, wherein a plurality of correction bodies21 can be arranged for each position.

FIG. 4 illustrates one variant for mechanically influencing a correctionbody 21. In this case, the correction body 21 is clamped in via twoactuator contact surfaces 11 extending parallel to the optical axis OA.By use of the actuators 24, via the actuator contact surfaces 11 amechanical force can be introduced perpendicularly to the optical axisOA into the correction body 21. It is likewise possible for theactuators 24 to move separately in the arrow direction 27 parallel tothe optical axis OA, as a result of which the force vectors 26 of theactuators 24 can act on the correction body 21 in opposite directionsand in a spatially offset manner. By use of the actuators 24 asmechanical influencing mechanism, it is thus possible to bring abouttensile and compressive forces, torques and also mechanical stresses ofarbitrary type statically and also dynamically in the correction body21, as a result of which the refractive index n of the dispersion medium3 and thus the dispersive behavior of the correction body 21 can becontrolled. It is conceivable for the correction body 21 to be fixedlyclamped in place and for the force impression to be effected only via anactuator 24. It is likewise possible for a change in the geometry of thedispersion medium 3 of the correction body 21 to be effected by use of acompressive or tensile force, as a result of which the optical pathlength L of the optical radiation within the correction body 21 canadditionally be influenced in a targeted manner. By way of example,electrical, hydraulic, pneumatic, thermal and magnetic actuators areconceivable as actuators 24. Furthermore, it is possible to usepiezo-actuators or acousto-optical modulators for modulating therefractive index n of the correction body 21 by use of mechanicalmechanisms. Furthermore, instead of actuators 24, it is also possible touse kinematics, in particular in the case of static influencing.

FIGS. 5A and 5B show two variants for influencing a correction body 21by use of electromagnetic fields and/or waves. In this case, in variantA, an electric field 13 is impressed within the dispersion medium 3 ofthe correction body 21 via two electrical contact pads 12. The electricfield 13 is controlled statically and dynamically via a voltagegenerator 25. The acting electric field 13 influences the dispersivebehavior of the correction body 21, as a result of which differentlongitudinal chromatic aberrations can be generated in a targetedmanner. By way of example, the refractive index n of the dispersionmedium 3 is modulated in a targeted manner via the electromagnetic field13 by use of linear effects, such as the Pockels effect or the Kerreffect, or by use of nonlinear optical effects. An alternativeembodiment is shown in variant B, wherein the electromagnetic radiation14 is impressed into the dispersion medium 3 within the correction body21 by use of an emitter 15.

FIGS. 6A and 6B illustrate two possible embodiments for setting theoptical path length L within a correction body. In this case, acorrection body 21′ in the form of an optical wedge 16 is illustrated inthe variant illustrated in FIG. 6A. The optical path length L within thecorrection body 21′ can be set by use of a displacement of the opticalwedge 16 perpendicularly to the optical axis OA. A correction body 21″in the form of an arrangement of two optical wedges 16 having a similarfunctional principle is illustrated in the variant shown in FIG. 6B. Inthis case, the two optical wedges 16 can be displaced, such that theyslide on their respective wedge surfaces 17 and thus cause a change inthe optical path length L. It is likewise conceivable that more than twooptical wedges 16 can be used.

FIG. 7 shows one embodiment for changing the spatial orientation andposition of correction bodies. In this case, optical lens elements L1,L2, L3, L4 are arranged along the optical axis OA within a correctionbody 21. The optical lens elements L1, L2, L3, L4 are positionedmechanically within the correction body 21 by use of a lens elementfixing 19 and a lens element mount 20. The dispersive behavior can beinfluenced firstly via an insertion and withdrawal of the optical lenselement L2 since a different longitudinal chromatic aberration existsdepending on the presence of the optical lens element L2 within thecorrection body 21. In this case, the optical lens element L2 can bechanged depending on the desired chromatic aberration, e.g. by use of alens element turret. The optical lens element L3 and the optical lenselement L4 are able to be mechanically freely arrangedthree-dimensionally and also rotationally about their axes in theidentified coordinate system in x-y-z-direction. In this case, the lenselements L3, L4 can assume different degrees of freedom separately byuse of their respective mechanism M3, M4. Firstly, the lens elements L3,L4 can be displaced by use of the mechanism M3, M4 separately inx-y-z-direction at their respective lens element mounts 20. Furthermore,each lens element L3, L4 can be rotated about the x-y and z-axis. Thelens elements L3, L4 can likewise be mutually tilted, if for example thedistances B′ and B are chosen to be different by use of the mechanismsM3, M4. By use of the different settable spatial orientations andpositions of the optical lens element L3 and of the optical lens elementL4, different longitudinal chromatic aberrations can thus be set andsimulated within the inspection device for masks 4.

FIG. 8 illustrates one variant for influencing the correction body byway of the gas pressure and the gas composition of the environment. Inthis case, the dispersion medium 3 of the correction body is situatedwithin a controllable gaseous environment 30. Via a purge gas unit 31,it is possible to influence the pressure P via a pressure regulation 32and the composition of the gas N via the gas component regulation 33. Byway of example, the gaseous environment uses nitrogen, which can beenriched in a controlled environment by use of noble gas additives. Onaccount of the action of the gaseous environment directly on thedispersion medium 3, the dispersive behavior of the correction body canbe influenced in a targeted manner.

FIGS. 9E, 9F, and 9G show three variants for influencing a correctionbody 21 by use of magnetic fields. In this case, in variant E, amagnetic field 41 can be impressed within the dispersion medium 3 of thecorrection body 21 by use of a permanent magnet 40. As further variantF, a coil 42 can be wound around the correction body 21 and thedispersion medium 3 thereof in order to build up a magnetic field 43.The coil 42 and the magnetic field 43 are influenced statically and alsodynamically via a voltage generator 25. An electromagnet 44 is likewiseconceivable as variant said electromagnetic being controlled via avoltage generator 25 for generating the magnetic field 45.

FIG. 10 illustrates one embodiment for influencing a correction body 21by use of a thermal element 50. In this case, the dispersion medium 3 ofthe correction body 21 is influenced thermally by a heating element 50,e.g. a Peltier element such that a temperature change arises within thedispersion medium 3. On account of the temperature dependence of thereflective index n, the dispersive behavior of the correction body 21 isinfluenced and a specific longitudinal chromatic aberration thus occurs.

FIG. 11 illustrates one variant for changing the spectral properties ofthe radiation source of the inspection device for masks 4 for thetargeted setting of a longitudinal chromatic aberration. In this case,the electromagnetic illumination radiation 6 of the spectral source 60can be spectrally influenced via an optical filter system 61. An opticalradiation source that is tunable in terms of its spectral property, e.g.a spectrally tunable LED, is conceivable a spectral source 60, as aresult of which it is possible to introduce wavelength-selectivelydifferent longitudinal chromatic aberrations in the inspection devicefor masks 4. Furthermore, a spectral source collective 63 comprisingvarious spectral sources 60 having different spectral properties, e.g.in the form of an array, is conceivable, wherein selectively onespectral source 60 from the spectral source collective 63 is introducedinto the beam path. Consequently, it is possible to simulate differentprojection exposure apparatuses and the longitudinal chromaticaberrations thereof within the inspection device for masks 4.

FIG. 12 shows, in a schematic illustration, a method for simulating alongitudinal chromatic aberration by use of recording a focus stack Fx.This involves recording for example 5 planes (F1, F2, F3, F4, F5) at adistance A of 100 nm in a defocused position. For this purpose, firstlythe camera and also the imaging device of the inspection device formasks can be adapted variably for each recording. The focusing of aprincipal plane L3 onto its corresponding focal plane F3 is illustratedby way of example. With knowledge of the line width of the usedradiation and the longitudinal chromatic aberration of projectionexposure apparatus to be simulated, from the individual images of thefocus stack Fx it is then possible to select those images which wouldcorrespond to the longitudinal chromatic aberration to be simulated. Thelongitudinal chromatic aberration can then be simulated by suitableinterpolation and, if appropriate, weighting of the images.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. The separationof various system components in the embodiments described above shouldnot be understood as requiring such separation in all embodiments.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results.

LIST OF REFERENCE SIGNS

-   -   1 Optical lens element    -   2 Electromagnetic radiation    -   3 Dispersion medium    -   4 Inspection device for masks    -   5 Radiation source    -   6 Illumination radiation    -   7 Illumination unit    -   8 Mask    -   9 Imaging device    -   10 Camera    -   11 Actuator contact surfaces    -   12 Electrical contact pad    -   13 Electric field    -   14 Electromagnetic radiation    -   15 Emitter    -   16 Optical wedge    -   17 Wage surfaces    -   19 Lens element fixing    -   20 Lens element mount    -   21 Correction body    -   22 Dispersion control mechanism    -   23 Control unit    -   24 Actuator    -   25 Voltage generator    -   26 Force vector    -   27 Direction parallel to the optical axis    -   A Distance of focal plane    -   OA Optical axis    -   F1 . . . F5 Focal plane    -   L1 . . . L3 Wavelength    -   N Gas composition    -   N Refractive index    -   P Pressure    -   Fx Focus stack    -   P1 . . . P6 Possible positions of the correction body    -   30 Gaseous environment    -   31 Purge gas unit    -   32 Pressure regulation    -   33 Gas component regulation    -   40 Permanent magnet    -   41 Magnetic field of permanent magnet    -   42 Coil    -   43 Magnetic field of coil    -   44 Electromagnet    -   45 Magnetic field of electromagnet    -   50 Heating element    -   60 Spectral source    -   61 Optical filter    -   63 Spectral source collective

1. A method for taking account of longitudinal chromatic aberrations ininspection devices for masks, the method comprising the following steps:recording a specific number of images having differently defocusedpositions, and selecting a subset of the images and simulating alongitudinal chromatic aberration of a projection exposure apparatus. 2.The method as claimed in claim 1, comprising an interpolation and/orweighting of the selected images.
 3. The method as claimed in claim 1,comprising a division into a main step size and a secondary step size.4. The method as claimed in claim 3, wherein for the main step size avalue of 0.3-2, in particular of 1, μm and for the secondary step size avalue of 10-150, in particular of 100, nm with respect to the mask arechosen.
 5. The method as claimed in claim 2, comprising a division intoa main step size and a secondary step size.
 6. The method as claimed inclaim 5, wherein for the main step size a value of 0.3-2, in particularof 1, μm and for the secondary step size a value of 10-150, inparticular of 100, nm with respect to the mask are chosen.
 7. The methodof claim 1 wherein recording the specific number of images havingdifferently defocused positions comprises measuring 5 planes at adistance of 100 nm between adjacent planes with respect to the mask. 8.The method of claim 7 wherein selecting the subset of the images andsimulating the longitudinal chromatic aberration of the projectionexposure apparatus comprises selecting images that are closest to animage that corresponds to an image in the projection exposure apparatususing knowledge of a line width of used radiation and the longitudinalchromatic aberration of the projection exposure apparatus.
 9. The methodof claim 8 wherein simulating the longitudinal chromatic aberration ofthe projection exposure apparatus comprises simulating the longitudinalchromatic aberration of the projection exposure apparatus byinterpolation of the images.
 10. The method of claim 9 whereinsimulating the longitudinal chromatic aberration of the projectionexposure apparatus comprises simulating the longitudinal chromaticaberration of the projection exposure apparatus by weightedinterpolation of the images.
 11. The method of claim 1 wherein recordingthe specific number of images comprises recording 5 images havingdifferently defocused positions.
 12. The method of claim 11 whereinrecording the specific number of images comprises recording the 5 imagesat a distance between adjacent planes with respect to the mask, and thedistance is in a range from 0.3 μm to 2 μm.
 13. The method of claim 12wherein selecting the subset of the images and simulating thelongitudinal chromatic aberration of the projection exposure apparatuscomprises selecting images that are closest to an image that correspondsto an image in the projection exposure apparatus using knowledge of aline width of used radiation and the longitudinal chromatic aberrationof the projection exposure apparatus.
 14. The method of claim 13 whereinsimulating the longitudinal chromatic aberration of the projectionexposure apparatus comprises simulating the longitudinal chromaticaberration of the projection exposure apparatus by interpolation of theimages.
 15. The method of claim 11 wherein recording the specific numberof images comprises recording the 5 images at a distance of betweenadjacent planes with respect to the mask, and the distance is in a rangefrom 10 nm to 150 nm.
 16. The method of claim 15 wherein selecting thesubset of the images and simulating the longitudinal chromaticaberration of the projection exposure apparatus comprises selectingimages that are closest to an image that corresponds to an image in theprojection exposure apparatus using knowledge of a line width of usedradiation and the longitudinal chromatic aberration of the projectionexposure apparatus.
 17. The method of claim 16, comprising simulatingthe longitudinal chromatic aberration of the projection exposureapparatus by interpolation of the images.
 18. The method of claim 1wherein selecting the subset of the images and simulating thelongitudinal chromatic aberration of the projection exposure apparatuscomprises selecting images that are closest to an image that correspondsto an image in the projection exposure apparatus using knowledge of aline width of used radiation and the longitudinal chromatic aberrationof the projection exposure apparatus.
 19. The method of claim 18,comprising simulating the longitudinal chromatic aberration of theprojection exposure apparatus by interpolation of the images.
 20. Themethod of claim 19, comprising simulating the longitudinal chromaticaberration of the projection exposure apparatus by weightedinterpolation of the images.